Magnetic resonance imaging apparatus and magnetic resonance imaging method of controlling image contrast

ABSTRACT

A magnetic resonance imaging apparatus includes a data acquisition unit and an image data generating unit. The data acquisition unit acquires MR signals for imaging by an imaging scan with a frequency-selective or slice-selective radio frequency intermediate pulse for controlling a contrast and a spoiler gradient magnetic field for suppressing unnecessary signal component after applying al least one of radio frequency excitation pulses. The image data generating unit generates image data based on the magnetic resonance signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a MRI (magnetic resonance imaging)apparatus and a magnetic resonance imaging method which excite nuclearspin of an object magnetically with a RF (radio frequency) signal havingthe Larmor frequency and reconstruct an image based on NMR (nuclearmagnetic resonance) signals generated due to the excitation, and moreparticularly, to a magnetic resonance imaging apparatus and a magneticresonance imaging method which perform imaging with applying a desiredRF pulse, such as a fat saturation pulse, for controlling an imagecontrast.

2. Description of the Related Art

Magnetic Resonance Imaging is an imaging method which excites nuclearspin of an object set in a static magnetic field with a RF signal havingthe Larmor frequency magnetically and reconstruct an image based on NMRsignals generated due to the excitation.

In the field of magnetic resonance imaging, the techniques to control animage contrast include the fat suppression methods. The fat suppressionmethods that has been widely used in general include the CHESS (chemicalshift selective) method, the SPIR (spectral presaturation with inversionrecovery) method (also referred as SPECIR method), and the STIR (shortTI inversion recovery) method.

Of the fat suppression methods, the CHESS method is referred as afrequency-selective fat suppression method since the method suppressesonly fat signals frequency-selectively using the fact that the resonancefrequencies of the water proton and the fat proton mutually differs by3.5 ppm (see, for example, Japanese Patent Application (Laid-Opendisclosure) No. 7-327960, Japanese Patent Application (Laid-Opendisclosure) No. 9-182729 and Japanese Patent Application (Laid-Opendisclosure) No. 11-299753). A CHESS pulse is applied as a RF pre-pulsein advance of data acquisition for imaging in the CHESS method.

The SPIR method is also a frequency-selective fat suppression methodwhich uses the difference in the resonance frequency between the waterproton and the fat proton (see, for example, Japanese Patent Application(Laid-Open disclosure) No. 2006-149583). In the SPIR method, a SPIRpulse that is a frequency-selective inversion RF pulse matched with aresonance frequency of fat signals is applied as a RF pre-pulse.

Meanwhile, the STIR method is a fat suppression method which uses adifference in T1 relaxation time between a fat signal and a water signaland a frequency-nonselective fat suppression method.

FIG. 1 is a time chart of the conventional pulse sequence under the FSE(fast spin echo) method with applying a frequency-selective fatsaturation pulse as a RF pre-pulse.

In FIG. 1, RF denotes RF pulses, Gss, Gro and Gpe denote axes to whichgradient magnetic field for slice selection, gradient magnetic field forRO (readout) and gradient magnetic field for PE (phase encode) areapplied respectively, ECHO denotes echo signals.

As shown in FIG. 1, an α° frequency-selective fat saturatio pulse RFc1for suppressing unnecessary signals from fat is applied as a RFpre-pulse prior to a FSE sequence for imaging. In addition, a spoilergradient magnetic field Gsp1 is applied in a gradient magnetic fielddirection for slice selection subsequently to the α° frequency-selectivefat saturation pulse RFc1.

In the FSE sequence, a flip pulse RFI1 with 90 degrees of FA (flipangle) is generally applied as a RF excitation pulse. In addition,plural refocus pulses RFI2, RFI3, RFI4, . . . are applied at an ETS(Echo Train Space) subsequently to the flip pulse RFI1. Each FA of therefocus pulses RFI2, RFI3, RFI4, . . . is generally set to 180 degrees.An interval between the flip pulse RFI1 and the first refocus pulse RFI2is set to ETS/2.

Meanwhile, a slice selection gradient magnetic field pulse Gss1corresponding to the flip pulse RFI1, and slice selection gradientmagnetic field pulses Gss2, Gss3, Gss4, . . . corresponding to therefocus pulses RFI2, RFI3, RFI4, . . . , respectively are applied. Theslice selection gradient magnetic field pulse Gss1 corresponding to theflip pulse RFI1 has a dephasing part. Each of the slice selectiongradient magnetic field pulses Gss2, Gss3, Gss4, . . . corresponding tothe refocus pulses RFI2, RFI3, RFI4, . . . , respectively has spoilergradient magnetic field parts on its both sides.

Further, readout gradient magnetic field pulses Gro2, Gro3, ... eachhaving a same area S are applied following the refocus pulses RFI2,RFI3, RFI4, ..., respectively. In addition, a readout gradient magneticfield pulse Gro1 for dephasing is applied following the flip pulse RFI1.The area of the readout gradient magnetic field pulse Gro1 for dephasingis set to be S/2 which is half of each area S of the readout gradientmagnetic field pulses Gro2, Gro3, ... applied subsequent to the refocuspulses RFI2, RFI3, RFI4, . . . .

Moreover, phase encode gradient magnetic field pulses Gpe1, Gpe2, Gpe3,Gpe4, . . . having reversed signs and equal areas are applied inintervals between respective applications of the refocus pulses RFI2,RFI3, RFI4, . . . .

In the foregoing pulse sequence, echo signals Echo1, Echo2, . . . aregenerated by application of the readout gradient magnetic field pulsesGro2, Gro3, . . . .

In recent years, the high magnetization technique in the MRI apparatushas been investigated and a high magnetic field apparatus have beenproduced. However, especially under a high magnetic field not less than3T, it is known that there is a B1 inhomogeneity problem that aninhomogeneity in a RF magnetic field increases due to attenuation of aRF pulse since a RF pulse having a shorter wavelength attenuates more ina living body while a resonance frequency becomes higher. The B1inhomogeneity is also referred as RF magnetic field inhomogeneity.

Consequently, an adequate fat suppression effect might not be achievedby simply applying a frequency-selective fat saturation pulse, such as aCHESS pulse, as a RF pre-pulse, depending on an imaging condition as inthe case of being under a high magnetic field.

This problem is common to an imaging with application of a RF pulse forcontrolling an image contrast, as well as a fat saturation pulse. Thatis, a desired image contrast could not be obtained simply by applying aRF pre-pulse for controlling an image contrast.

On the other hand, an imaging with application of a RF pre-pulse has aproblem with increasing an imaging time. Especially in multi-sliceimaging, the problem is that the minimum TR (repetition time) isincreased by application of a RF pre-pulse and to increase the number ofslices becomes difficult. Note that, the minimum TR is a TR for imaginga specific slice set consisting of multiple slices.

SUMMARY OF THE INVENTION

The present invention has been made in light of the conventionalsituations, and it is an object of the present invention to provide amagnetic resonance imaging apparatus and a magnetic resonance imagingmethod which make it possible to control an image contrastsatisfactorily by applying an RF pulse for a desired purpose such as fatsuppression with a shorter imaging period.

The present invention provides a magnetic resonance imaging apparatuscomprising: a data acquisition unit configured to acquire magneticresonance signals for imaging by an imaging scan with applying afrequency-selective or slice-selective radio frequency intermediatepulse for controlling a contrast and a spoiler gradient magnetic fieldfor suppressing unnecessary signal component after applying al least oneradio frequency excitation pulse to be applied for acquiring themagnetic resonance signals, the spoiler gradient magnetic field beingsubsequent to the radio frequency intermediate pulse; and an image datagenerating unit configured to generate image data based on the magneticresonance signals, in an aspect to achieve the object.

The present invention also provides a magnetic resonance imagingapparatus comprising: a data acquisition unit configured to acquiremagnetic resonance signals for imaging by an imaging scan with applyinga frequency-selective or slice-selective fat saturation pulse forcontrolling a contrast and a spoiler gradient magnetic field forsuppressing unnecessary signal component after applying al least one ofradio frequency excitation pulses to be applied for acquiring themagnetic resonance signals, the spoiler gradient magnetic field beingsubsequent to the fat saturation pulse; and an image data generatingunit configured to generate image data based on the magnetic resonancesignals, in an aspect to achieve the object.

The present invention also provides a magnetic resonance imaging methodcomprising: acquiring magnetic resonance signals for imaging by animaging scan with applying a frequency-selective or slice-selectiveradio frequency intermediate pulse for controlling a contrast and aspoiler gradient magnetic field for suppressing unnecessary signalcomponent after applying al least one of radio frequency excitationpulses to be applied for acquiring the magnetic resonance signals, thespoiler gradient magnetic field being subsequent to the radio frequencyintermediate pulse; and generating image data based on the magneticresonance signals, in an aspect to achieve the object.

The magnetic resonance imaging apparatus and the magnetic resonanceimaging method according to the present invention as described abovemake it possible to control an image contrast satisfactorily by applyingan RF pulse for a desired purpose such as fat suppression with a shorterimaging period.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a time chart of the conventional pulse sequence under the FSE(fast spin echo) method with applying a frequency-selective fatsaturation pulse as a RF pre-pulse;

FIG. 2 is a block diagram showing a magnetic resonance imaging apparatusaccording to an embodiment of the present invention;

FIG. 3 is a functional block diagram of the computer shown in FIG. 2;

FIG. 4 is a time chart showing an example of FSE sequence, with applyingtwo fat-saturation pulses as a RF pre-pulse and a RF intermediate pulse,which is set by the imaging condition setting unit shown in FIG. 3;

FIG. 5 is a time chart showing an example of DWI (diffusion weightedimaging) sequence, with applying two fat-saturation pulses as a RFpre-pulse and a RF intermediate pulse, which is set by the imagingcondition setting unit shown in FIG. 3; and

FIG. 6 is a flowchart showing a procedure for acquiring an image of theobject by imaging with applying a RF intermediate pulse with themagnetic resonance imaging apparatus shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A magnetic resonance imaging apparatus and a magnetic resonance imagingmethod according to embodiments of the present invention will bedescribed with reference to the accompanying drawings.

(Configuration and Function)

FIG. 2 is a block diagram showing a magnetic resonance imaging apparatusaccording to an embodiment of the present invention.

A magnetic resonance imaging apparatus 20 includes a static field magnet21 for generating a static magnetic field, a shim coil 22 arrangedinside the static field magnet 21 which is cylinder-shaped, a gradientcoil 23 and RF coils 24.

The magnetic resonance imaging apparatus 20 also includes a controlsystem 25. The control system 25 includes a static magnetic field powersupply 26, a gradient power supply 27, a shim coil power supply 28, atransmitter 29, a receiver 30, a sequence controller 31 and a computer32. The gradient power supply 27 of the control system 25 includes anX-axis gradient power supply 27 x, a Y-axis gradient power supply 27 yand a Z-axis gradient power supply 27 z. The computer 32 includes aninput device 33, a display unit 34, a operation unit 35 and a storageunit 36.

The static field magnet 21 communicates with the static magnetic fieldpower supply 26. The static magnetic field power supply 26 supplieselectric current to the static field magnet 21 to get the function togenerate a static magnetic field in a imaging region. The static fieldmagnet 21 includes a superconductivity coil in many cases. The staticfield magnet 21 gets current from the static magnetic field power supply26 which communicates with the static field magnet 21 at excitation.However, once excitation has been made, the static field magnet 21 isusually isolated from the static magnetic field power supply 26. Thestatic field magnet 21 may include a permanent magnet which makes thestatic magnetic field power supply 26 unnecessary.

The static field magnet 21 has the cylinder-shaped shim coil 22coaxially inside itself. The shim coil 22 communicates with the shimcoil power supply 28. The shim coil power supply 28 supplies current tothe shim coil 22 so that the static magnetic field becomes uniform.

The gradient coil 23 includes an X-axis gradient coil 23 x, a Y-axisgradient coil 23 y and a Z-axis gradient coil 23 z. Each of the X-axisgradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z which is cylinder-shaped is arranged inside thestatic field magnet 21. The gradient coil 23 has also a bed 37 in thearea formed inside it which is an imaging area. The bed 37 supports anobject P. The RF coils 24 include a whole body coil (WBC: whole bodycoil), which is built in the gantry, for transmission and reception ofRF signals and local coils, which are arranged around the bed 37 or theobject P, for reception of RF signals.

The gradient coil 23 communicates with the gradient power supply 27. TheX-axis gradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z of the gradient coil 23 communicate with the X-axisgradient power supply 27 x, the Y-axis gradient power supply 27 y andthe Z-axis gradient power supply 27 z of the gradient power supply 27respectively.

The X-axis gradient power supply 27 x, the Y-axis gradient power supply27 y and the Z-axis gradient power supply 27 z supply currents to theX-axis gradient coil 23 x, the Y-axis gradient coil 23 y and the Z-axisgradient coil 23 z respectively so as to generate gradient magneticfields Gx, Gy and Gz in the X, Y and Z directions in the imaging area.

The RF coils 24 communicate with the transmitter 29 and/or the receiver30. The transmission RF coil 24 has a function to transmit a RF signalgiven from the transmitter 29 to the object P. The reception RF coil 24has a function to receive a MR signal generated due to an nuclear spininside the object P which is excited by the RF signal to give to thereceiver 30.

The sequence controller 31 of the control system 25 communicates withthe gradient power supply 27, the transmitter 29 and the receiver 30.The sequence controller 31 has a function to store sequence informationdescribing control information needed in order to make the gradientpower supply 27, the transmitter 29 and the receiver 30 generategradient magnetic fields Gx, Gy and Gz in the X, Y and Z directions anda RF signal by driving the gradient power supply 27, the transmitter 29and the receiver 30 according to a predetermined stored sequence. Theabove-described control information includes motion control informationsuch as intensity, impression period and impression timing of the pulseelectric current which should be impressed to the gradient power supply27.

The sequence controller 31 is also configured to give raw data to thecomputer 32. The raw data is complex data obtained through the detectionof a NMR signal and A/D conversion to the NMR signal detected in thereceiver 30.

The transmitter 29 has a function to give a RF signal to the RF coil 24in accordance with control information provided from the sequencecontroller 31. The receiver 30 has a function to generate raw data whichis digitized complex number data by detecting a MR signal given from theRF coil 24 and performing predetermined signal processing and A/Dconverting to the detected MR signal. The receiver 30 also has afunction to give the generated raw data to the sequence controller 31.

The computer 32 performs various functions in the operation unit 35 byexecuting some programs stored in the storage unit 36 of the computer32. Alternatively, some specific circuits having various functions maybe provided with the magnetic resonance imaging apparatus 20 instead ofusing some of the software programs.

FIG. 3 is a functional block diagram of the computer 32 shown in FIG. 2.The computer 32 functions as an imaging condition setting unit 40, asequence controller control unit 41, a k-space database 42, an imagereconstruction unit 43, an image database 44 and an image processingunit 45 by executing certain software programs.

The imaging condition setting unit 40 has a function to set a pulsesequence with applying a frequency-selective or slice-selective RF pulsesuch as a fat-saturation pulse for controlling an image contrast and animaging condition based on instruction from the input device 33, and toprovide the set imaging condition to the sequence controller controlunit 41. The RF pulse for controlling an image contrast is set as a RFintermediate pulse at least in an imaging sequence for acquiring NMRsignals for imaging. Specifically, an application timing of a RFintermediate pulse for controlling an image contrast is set after at anearliest application of a RF excitation pulse for acquiring imagingdata.

For example, when a timing for applying a RF intermediate pulse isbefore a timing for acquiring data on the center of the K spacedetermined according to a TE (echo time) and an ETS, a more satisfactorycontrast control can be performed by providing the effect of the RFintermediate pulse with the data, having the most effect on an imagecontrast, on the center of the K space. Therefore, a RF intermediatepulse can be applied multiple times at both timings before acquisitionof all imaging data and before acquisition of the data on the center ofthe K space. However, the application of a RF intermediate pulse only atthe timing before acquisition of the data on the center of the K spaceleads to shortening an imaging time.

Moreover, when it is preferable to control an image contrast moresatisfactorily or intricately, for example, a frequency-selective orslice-selective RF pre-pulse for controlling an image contrast can beapplied prior to an imaging sequence, i.e., before application of a RFexcitation pulse for acquiring imaging data, as needed. On the contrary,when there is a need for shortening an imaging time, an imagingcondition in which a RF intermediate pulse is applied without applying aRF pre-pulse may be set.

In addition, the pulse sequence is set so that a spoiler gradientmagnetic field pulse for suppressing transverse magnetization signalcomponent from an unnecessary metabolite such as fat is applied aftereach application of a RF intermediate pulse and a RF pre-pulse.

A RF contrast control pulse applied as a RF intermediate pulse or a RFpre-pulse can be an arbitrary pulse as long as it is either aslice-selective pulse as applied in a slice that is different from animaging slice or a frequency-selective pulse as applied to a matterhaving a resonance frequency that is different from that of NMR signalsacquired for imaging. Therefore, examples of a RF contrast control pulseinclude a water-selective excitation pulse, a fat saturation pulse, asaturation pulse, a spin labeling pulse, a MTC (magnetization transfercontrast) pulse and a SORS (slice-selective off-resonance sinc) pulse.Therefore, examples of a RF contrast control pulse include awater-selective excitation pulse, a fat saturation pulse, a saturationpulse, a spin labeling pulse, a MTC (magnetization transfer contrast)pulse and a SORS (slice-selective off-resonance sinc pulse).

The water-selective excitation pulse is a RF contrast control pulse forselectively exciting water, and the fat saturation pulse is a RFcontrast control pulse for suppressing signals from fat. The saturationpulse is a RF contrast control pulse for saturating spins in a desiredmatter to suppress signals from the desired matter and is applied beforeapplication of a dephasing gradient magnetic field. The spin labelingpulse is a RF contrast control pulse for tagging a moving object flowinginto an imaging section. The MTC pulse is a RF contrast control pulsefor saturating magnetization of protons in bound water to suppresssignals from parenchyma organs. The MTC pulse applied together with aslice selection gradient magnetic field is referred as the SORS.

A RF intermediate pulse and a RF pre-pulse can be RF contrast controlpulses of the same kind and alternatively RF contrast control pulses ofmutually different kinds. When a RF intermediate pulse and a RFpre-pulse are the same type of RF contrast control pulses, a contrastcontrol can be performed more satisfactorily without increasing animaging time. On the contrary, when a RF intermediate pulse and a RFpre-pulse are mutually different types of RF contrast control pulses,several types of contrast controls can be performed without increasingan imaging time.

In addition, an imaging condition can be set so that multiple RFintermediate pulses and/or multiple RF pre-pulses are applied. Even inthose cases, multiple RF intermediate pulses can be set to the same typeor different types of RF contrast control pulses. Similarly, multiple RFpre-pulses can be set to the same type or different types of RF contrastcontrol pulses.

Note that, a fat saturation pulse is considered to be in the largestdemands as a RF contrast control pulse. Therefore, an example ofapplying two fat saturation pulses as a single RF pre-pulse and a singleRF intermediate pulse is described hereinafter. However, the same holdstrue for applying another RF contrast control pulse as a single RFpre-pulse, one of multiple RF pre-pulses, a single RF intermediate pulseor one of multiple RF intermediate pulses.

FIG. 4 is a time chart showing an example of FSE sequence, with applyingtwo fat-saturation pulses as a RF pre-pulse and a RF intermediate pulse,which is set by the imaging condition setting unit 40 shown in FIG. 3.

In FIG. 4, RF denotes RF pulses while Gss, Gro and Gpe denote axes towhich gradient magnetic field for slice selection, gradient magneticfield for readout and gradient magnetic field for phase encode arerespectively applied. ECHO denotes echo signals.

As shown in FIG. 4, an α1° frequency-selective (chemical shiftselective) fat saturation pulse RFc1 for suppressing unnecessary signalsfrom fat is applied as a RF pre-pulse prior to a FSE sequence forimaging. Further, a spoiler gradient magnetic field Gsp1 is applied in aslice selection gradient magnetic field direction, following the α1°frequency-selective fat saturation pulse RFc1. The FA of the fatsaturation pulse RFc1 is set to about α1°=90° to 130° according to arequired fat suppression effect.

In the FSE sequence, a flip pulse RFI1 with a 90 degrees of FA isgenerally applied as a RF excitation pulse. Further, multiple refocuspulses RFI2, RFI3, RFI4, . . . are applied at an ETS, following the flippulse RFI1. The FA of the refocus pulses RFI2, RFI3, RFI4, . . . isgenerally set to 180 degrees. An interval between the flip pulse RFI1and the first refocus pulse RFI2 is set to ETS/2.

Moreover, an α2° frequency-selective fat saturation pulse RFc2 isapplied as a RF intermediate pulse at an arbitrary position on the FSEsequence after application of the flip pulse RFI1. FIG. 4 shows anexample that the α2° frequency-selective fat saturation pulse RFc2 isapplied between the first refocus pulse RFI2 and the second refocuspulse RFI3. However, the α2° frequency-selective fat saturation pulseRFc2 may be applied after application of an arbitrary refocus pulse.

Note that, the α2° frequency-selective fat saturation pulse RFc2 may beapplied between the flip pulse RFI1 and the first refocus pulse RFI2.However, the interval between the refocus pulses is longer than theinterval between the flip pulse RFI1 and the first refocus pulse RFI2.Therefore, when the frequency-selective fat saturation pulse RFc2 isapplied between the refocus pulses, it is highly possible that the pulselength of the frequency-selective fat saturation pulse RFc2 can be setto a desired length. Therefore, it is preferable to apply thefrequency-selective fat saturation pulse RFc2 between the refocus pulsesto obtain a desired contrast easily.

In addition, it is preferable to determine the FA of the fat saturationpulse RFc2 so that the contrast control effect can be constantlymaintained according to the FA of the refocus pulses RFI2, RFI3, RFI4, .. . . For example, if the FA of the refocus pulses RFI2, RFI3, RFI4, . .. is 180°, it is preferable to set the FA of the fat saturation pulseRFc2 to α2°=180°. In this case, since a magnetization in a fat regioninverts by 180° with respect to a magnetization in a region other thanfat by application of the fat saturation pulse RFc2, the suppressedmagnetization state in the fat region is constantly maintained eventhough the 180° refocus pulse is applied repeatedly.

For example, an adiabatic pulse can be used for the α2°frequency-selective fat saturation pulse RFc2 as well as the α1°frequency-selective fat saturation pulse RFc1.

Thus, the α1° frequency-selective fat saturation pulse RFc1 functions asthe first fat saturation pulse and the α2° frequency-selective fatsaturation pulse RFc2 functions as the second fat saturation pulse.

Meanwhile, a slice selection gradient magnetic field pulse Gsslcorresponding to the flip pulse RFI1, and slice selection gradientmagnetic field pulses Gss2, Gss3, Gss4, . . . respectively correspondingto the refocus pulses RFI2, RFI3, RFI4, . . . , are applied. The sliceselection gradient magnetic field pulse Gss1 corresponding to the flippulse RFI1 has a dephasing part. Each of the slice selection gradientmagnetic field pulses Gss2, Gss3, Gss4, . . . respectively correspondingto the refocus pulses RFI2, RFI3, RFI4, . . . , has a spoiler gradientmagnetic field parts on its both sides.

Further, a readout gradient magnetic field pulse (Gro3, . . . in thecase of FIG. 4) having the same area S is applied following each of therefocus pulses (RFI3, RFI4, . . . in the case of FIG. 4) except for arefocus pulse applied just before application of the α2° fat saturationpulse RFc2. In addition, a readout gradient magnetic field pulse Gro1for dephasing is applied following the flip pulse RFI1. The area of thereadout gradient magnetic field pulse Gro1 for dephasing is S/2 which ishalf of each area S of the readout gradient magnetic field pulses (Gro3. . . in the case of FIG. 4) applied subsequent to the refocus pulses(RFI3, RFI4, . . . in the case of FIG. 4).

Further, an arbitrary number of readout gradient magnetic field pulsesare applied between the refocus pulse (RFI2 in the case of FIG. 4)applied just before application of the α2° fat saturation pulse RFc2 andthe refocus pulse (RFI3 in the case of FIG. 4) applied just afterapplication of the α2° fat saturation pulse RFc2. FIG. 4 shows anexample in which two readout gradient magnetic field pulses Gsp2 andGsp3 are applied before and after the α2° fat saturation pulse RFc2. Thetotal area of an arbitrary number of readout gradient magnetic fieldpulses (Gsp2 and Gsp3 in the case of FIG. 4) applied before and afterthe α2° fat saturation pulse RFc2 is set to be the same as each area Sof the readout gradient magnetic field pulses (Gro3, . . . in the caseof FIG. 4) applied subsequent to other refocus pulses (RFI3, RFI4, . . .in the case of FIG. 4). Therefore, as shown in FIG. 4, when the area ofthe readout gradient magnetic field pulse Gsp2 applied before the α2°fat saturation pulse RFc2 is S1, the area of the readout gradientmagnetic field pulse Gsp3 applied after the α2° fat saturation pulseRFc2 becomes S-S1.

That is, to avoid applying a readout gradient magnetic field pulse atthe timing of application of the α2° fat saturation pulse RFc2, it canbe thought that the readout gradient magnetic field pulse that should beoriginally applied in the interval between the refocus pulses RFI2 andRFI3 where the α2° fat saturation pulse RFc2 is applied is divided withthe constant area to be set before and after the α2° fat saturationpulse RFc2. By doing so, the readout gradient magnetic field pulse Gsp3applied after the α2° fat saturation pulse RFc2 can function as aspoiler gradient magnetic field pulse corresponding to the α2° fatsaturation pulse RFc2 by simply adjusting an imaging condition.Therefore, it is preferable to determine the area S-S1 of the readoutgradient magnetic field pulse Gsp3 applied after the α2° fat saturationpulse RFc2 so as to function adequately as a spoiler gradient magneticfield pulse.

However, since the area of the readout gradient magnetic field pulseGsp3 applied after the α2° fat saturation pulse RFc2 has an upper limit,a sufficient area might not be acquired. Accordingly, a spoiler gradientmagnetic field pulse having an arbitrary area can be set in either aslice selection gradient magnetic field direction or a phase encodegradient magnetic field direction, or in both the directions. Thisallows setting an intensity of a spoiler gradient magnetic field pulsearbitrarily. In this case, if a pre-spoiler gradient magnetic fieldpulse having the same area as that of the spoiler gradient magneticfield pulse and the polarity opposite to that of the spoiler gradientmagnetic field pulse is applied in application axial directions of thespoiler gradient magnetic field pulse before the α2° fat saturationpulse RFc2, a dephasing amount by the spoiler gradient magnetic fieldpulse can be cancelled so that echo signals from metabolites to beobserved can be successfully acquired.

Note that, when a spoiler gradient magnetic field pulse is set in aslice selection gradient magnetic field direction or a phase encodegradient magnetic field direction, a readout gradient magnetic fieldpulse Gsp2 having the area S may be applied only before the α2° fatsaturation pulse RFc2 with setting the area S-S1 of the readout gradientmagnetic field pulse Gsp3, applied as a spoiler gradient magnetic fieldpulse after the α2° fat saturation pulse RFc2, to zero. Meanwhile, areadout gradient magnetic field pulse Gsp3 having the area S may beapplied only after the α2° fat saturation pulse RFc2 without applyingthe readout gradient magnetic field pulse Gsp2 before the α2° fatsaturation pulse RFc2 whether a spoiler gradient magnetic field pulse isapplied in a direction other than the readout gradient magnetic fielddirection or not. That is, the readout gradient magnetic field pulsethat should be applied in the interval between the refocus pulses RFI2and RFI3 where the α2° fat saturation pulse RFc2 is applied may not benecessarily divided.

Moreover, phase encode gradient magnetic field pulses Gpe3, Gpe4, ...having opposite signs and a equal area are applied in the intervalsamong respective applications of the refocus pulses RFI2, RFI3, RFI4,.... However, it is also possible to remove the phase encode gradientmagnetic field pulses applied in the interval between the refocus pulsesRFI2 and RFI3 where the α2° fat saturation pulse RFc2 is applied, and toset the foregoing pre-spoiler gradient magnetic field pulse and theforegoing spoiler gradient magnetic field pulse.

FIG. 4 shows an example that the pre-spoiler gradient magnetic fieldpulse Gsp4 and the spoiler gradient magnetic field pulse Gsp5 that havethe same area and mutually opposite polarities are set in the phaseencode gradient magnetic field direction before and after the α2° fatsaturation pulse RFc2. Then, unnecessary signal component from fat issuppressed by application of the spoiler gradient magnetic field pulseGsp5 while a dephasing amount by the spoiler gradient magnetic fieldpulse Gsp5 can be cancelled by application of the pre-spoiler gradientmagnetic field pulse Gsp4. Consequently, NMR signals from a desiredmetabolite to be observed other than fat can be successfully extracted.Note that, a pre-spoiler gradient magnetic field pulse and a spoilergradient magnetic field pulse may be applied in the slice selectiongradient magnetic field direction as described above.

In the foregoing pulse sequence, echo signals are generated byapplication of the readout gradient magnetic field pulse Gro3, . . .during the intervals among the refocus pulses RFI3, RFI4, . . . wherethe α2° fat saturation pulse RFc2 is not applied.

Meanwhile, not only a FSE sequence but an arbitrary pulse sequence basedon another imaging technique can be used as an imaging sequence. Thatis, application of a RF intermediate pulse can be set after applicationof a RF excitation pulse such as a flip pulse, a flop pulse or a refocuspulse. In addition, application of a RF pre-pulse can be set beforeapplication of a flip pulse or a flop pulse as needed.

In this case, gradient magnetic field pulses must be determined so thatno gradient magnetic field pulse is applied at a timing of applicationof the RF intermediate pulse, and a gradient magnetic field pulse suchas a readout gradient magnetic field pulse applied just after the RFintermediate pulse can be used as a spoiler gradient magnetic fieldpulse. Meanwhile, when a pulse available as a spoiler gradient magneticfield pulse does not exist after the RF intermediate pulse, a spoilergradient magnetic field pulse and a pre-spoiler gradient magnetic fieldpulse can be set before and after the RF intermediate pulse in anarbitrary axis direction or a plurality of arbitrary axis directions ofthe readout gradient magnetic field direction, the phase encode gradientmagnetic field direction and the slice selection gradient magnetic fielddirection.

Then, data can be more successfully acquired with suppressing signalsfrom an unnecessary metabolite such as fat to control the contrast byapplication of a readout gradient magnetic field pulse after the RFintermediate pulse.

FIG. 5 is a time chart showing an example of DWI (diffusion weightedimaging) sequence, with applying two fat-saturation pulses as a RFpre-pulse and a RF intermediate pulse, which is set by the imagingcondition setting unit 40 as shown in FIG. 3.

In FIG. 5, RF denotes RF pulses while Gss and Gro denote axes to whichgradient magnetic field for slice selection and gradient magnetic fieldfor readout are respectively applied. ECHO denotes echo signals. Notethat, explanations of gradient magnetic field for phase encode and eachpulse similarly to a pulse shown in FIG. 4 are omitted.

As shown in FIG. 5, an α1° frequency-selective fat saturation pulse RFc1is applied as a RF pre-pulse prior to a DWI sequence for imaging.

For example, the DWI sequence is a sequence derived by adding a MPG(motion probing gradient) pulse to a SE-EPI sequence based on the SE(Spin Echo) method and the EPI (echo planar imaging) method. That is, a180° RF pulse RFI2 is applied after TE/2 following application of a 90°RF excitation pulse RFI1. In addition, slice selection gradient magneticfield pulses Gss1 and Gss2 are applied together respectively with the90° RF excitation pulse RFI1 and the 180° RF pulse RFI2. Then, echosignals can be acquired continuously after a TE from application of the90° RF excitation pulse RFI1 by applications of readout gradientmagnetic field pulses Gepi-ro set based on the EPI method.

Moreover, an arbitrary number of MPG pulses are applied in arbitrarygradient magnetic field directions arbitrarily set depending on animaging purpose. For example as shown in FIG. 5, the first positive MPGpulse MPG1 and the second negative MPG pulse MPG2 are applied in thereadout gradient magnetic field direction between the 90° RF excitationpulse RFI1 and the 180° RF pulse RFI2, and the third positive MPG pulseMPG3 is applied in the readout gradient magnetic field direction betweenthe 180° RF pulse RFI2 and the readout gradient magnetic field pulsesGepi-ro based on the EPI method.

Moreover, an α2° frequency-selective fat saturation pulse RFc2 can beapplied as a RF intermediate pulse at an arbitrary position on the DWIsequence after application of the 90° RF excitation pulse RFI1. Forexample as shown in FIG. 5, if the α2° frequency-selective fatsaturation pulse RFc2 is applied between the first positive MPG pulseMPG1 and the second negative MPG pulse MPG2, the second negative MPGpulse MPG2 can be used as a spoiler gradient magnetic field pulse. Inaddition, if the α2° frequency-selective fat saturation pulse RFc2 isapplied before acquisition of the first echo signal for imaging, theeffect of the α2° frequency-selective fat saturation pulse RFc2 can beprovided to all echo signals.

Further, even when a MPG pulse is not set in an appropriate position inthe DWI sequence before having set the α2° fat saturation pulse RFc2, aMPG pulse can be intentionally set so as to be used as a spoilergradient magnetic field pulse in an appropriate position after the α2°fat saturation pulse RFc2 by keeping a total area of all MPG pulsesconstant. Moreover, a pair of a pre-spoiler gradient magnetic fieldpulse and a spoiler gradient magnetic field pulse can be set inarbitrary axis directions, e.g., axes where no MPG pulse is applied.

In addition, when the α2° fat saturation pulse RFc2 is applied betweenthe 180° RF pulse RFI2 and the readout gradient magnetic field pulsesGepi-ro based on the EPI method, echo signals acquired by applicationsof the readout gradient magnetic field pulses Gepi-ro can also achievethe fat suppression effect by the α2° fat saturation pulse RFc2. In thiscase, the third MPG pulse MPG3 can be used as a spoiler gradientmagnetic field pulse.

Now, other functions of the computer 32 will be described.

The sequence controller control unit 41 has a function for controllingthe driving of the sequence controller 31 by giving the imagingcondition including the pulse sequence, acquired from the imagingcondition setting unit 40, to the sequence controller 31 based oninformation from the input device 33 or another element. In addition,the sequence controller control unit 41 has a function for receiving rawdata from the sequence controller 31 and arranging the raw data to kspace formed in the k-space database 42. Therefore, the k-space database42 stores the raw data generated by the receiver 30 as k space data.

The image reconstruction unit 43 has a function for reconstructing imagedata by obtaining the k-space data from the k-space database 42 andperforming image reconstruction processing including FT (Fouriertransform) of the k-space data and a function for writing the generatedimage data to the image database 44. Therefore, the image database 44stores the image data reconstructed by the image reconstruction unit 43.

The image processing unit 45 has a function for generatingtwo-dimensional image data for display by performing necessary imageprocessing to the image data read form the image database 44 anddisplaying the generated image data on the display unit 34. (Operationand action)

Now, the operation and action of a magnetic resonance imaging apparatus20 will be described.

FIG. 6 is a flowchart showing a procedure for acquiring an image of theobject P through imaging by applying a RF intermediate pulse with themagnetic resonance imaging apparatus 20 as shown in FIG. 2. Thereference numerals each including a character S with a number in FIG. 6indicate respective steps of the process.

In step S1, an imaging condition including a pulse sequence, as shown inFIG. 4 or FIG. 5, with applying a RF intermediate pulse for controllinga contrast is set in the image condition setting unit 40 based oninformation input from the input device 33. In addition, a RF pre-pulseis added as needed.

Subsequently, in step S2, an imaging scan is performed according to theset imaging condition for data acquisition

For that purpose, the object P is placed on the bed 37, and a staticmagnetic field is generated at an imaging area of the magnet 21 (asuperconducting magnet) for static magnetic field as excited by thestatic-magnetic-field power supply 26. Further, the shim-coil powersupply 28 supplies current to the shim coil 22, thereby uniformizing thestatic magnetic field generated at the imaging area.

The input device 33 sends instructions for starting an imaging scan tothe sequence controller control unit 41. The sequence controller controlunit 41 supplies a pulse sequence by applying a RF intermediate pulse,received from the imaging condition setting unit 40, to the sequencecontroller 31. Therefore, the sequence controller 31 drives the gradientpower supply 27, the transmitter 29, and the receiver 30 in accordancewith the pulse sequence received from the sequence controller controlunit 41, thereby generating a gradient magnetic field at an imaging areahaving the set object P, and further generating RF signals from the RFcoil 24.

Consequently, the RF coil 24 receives NMR signals generated due tonuclear magnetic resonance in the object P. Then, the receiver 30receives the NMR signals from the RF coil 24 and generates raw datawhich is digital data of the NMR signals by A/D conversion subsequent tosome necessary signal processing. The receiver 30 supplies the generatedraw data to the sequence controller 31. The sequence controller 31supplies the raw data to the sequence controller control unit 41. Thesequence controller control unit 41 arranges the raw data as k-spacedata to the k space formed in the k-space database 42.

Subsequently, in step S3, the image reconstruction unit 43 reads thek-space data from the k-space database 42 and performs imagereconstruction processing including Fourier transform to the readk-space data, thereby reconstructing image data. The generated imagedata is written into the image database 44.

Subsequently, in step S4, the image processing unit 45 reads the imagedata form the image database 44 and performs necessary image processingof the image data, thereby generating two-dimensional image data fordisplaying. The generated display image data is displayed on the displayunit 34. Since the image data is acquired according to an imagingcondition by applying a RF intermediate pulse as shown in FIG. 4 or FIG.5, for controlling the contrast, the image data can be acquired in aless imaging time. Moreover, if a RF pre-pulse for controlling thecontrast is applied, a more satisfactory fat suppression effect can beachieved in the image data.

That is, the foregoing magnetic resonance imaging apparatus 20 is anapparatus which acquires image data by applying a RF intermediate pulse,such as a fat saturation pulse, for controlling a contrast and a spoilergradient magnetic field pulse during execution of an imaging sequence.

Consequently, according to the foregoing magnetic resonance imagingapparatus 20, the first TR can be shortened since a RF pre-pulse is notnecessarily required. Especially in the case of a multi slice imaging,the minimum TR for performing imaging a specific slice set can beshortened. For this reason, the number of slices can be increased inmulti slice imaging with performing a contrast control such as fatsuppression.

Moreover, if the same kind of contrast control pulse such as a RFintermediate pulse is applied as a RF pre-pulse in the magneticresonance imaging apparatus 20, more satisfactory contrast control canbe performed. For example, if fat saturation pulses are applied as a RFpre-pulse and a RF intermediate pulse, an adequate fat suppressioneffect can be achieved. Meanwhile, if a different kind of contrastcontrol pulse from that of a RF intermediate pulse is applied as a RFpre-pulse, various image contrast controls can be performed depending onan imaging purpose.

Since a chemical shift amount increases especially in a high magneticfield apparatus exceeding 3T, improvement of an image contrast can beexpected by application of the foregoing RF intermediate pulse.

Note that, multichannelizing of a RF transmission pulse that transmits aRF transmission pulse formed by using multiple RF element coils has beendesigned. For that reason, if a RF pre-pulse and/or a RF intermediatepulse are transmitted by using multiple RF element coils, profiles ofthe RF pre-pulse and/or the RF intermediate pulse in a frequencydirection can be stabilized. Consequently, a more satisfactory contrastcontrol can be performed.

1. A magnetic resonance imaging apparatus comprising: a data acquisitionunit configured to perform an imaging scan based on a Fast Spin Echosequence, the imaging scan including applying at least one radiofrequency excitation pulse for acquiring magnetic resonance signals;applying a plurality of refocus pulses after each of the radio frequencyexcitation pulses; for controlling a contrast, applying one or morefrequency-selective or slice-selective radio frequency intermediatepulses, wherein each of the one or more radio frequency intermediatepulses is separately applied between at least one respective pair ofsuccessive refocus pulses, and the one or more frequency-selective radiofrequency intermediate pulses are radio frequency pulses applied to amatter having a resonance frequency different from that of NMR signalsacquired for imaging and the one or more slice-selective radio frequencyintermediate pulses are radio frequency pulses applied in a slicedifferent from an imaging slice; for suppressing a signal component,applying a spoiler gradient magnetic pulse subsequent to each of theradio frequency intermediate pulses; and acquiring magnetic resonancesignals for imaging; and an image data generating unit configured togenerate image data based on the magnetic resonance signals.
 2. Amagnetic resonance imaging apparatus of claim 1, wherein the one or moreradio frequency intermediate pulses include a fat saturation pulse.
 3. Amagnetic resonance imaging apparatus of claim 1, wherein the one or moreradio frequency intermediate pulses include at least one of a waterselective excitation pulse, a saturation pulse for suppressing signalsfrom a respective matter, a spin labeling pulse, a magnetizationtransfer contrast pulse for suppressing signals from by saturating amagnetization of protons in bound water and a slice-selectiveoff-resonance sinc pulse, which is a magnetization transfer contrastpulse applied with a slice selection gradient magnetic field.
 4. Amagnetic resonance imaging apparatus of claim 1, wherein said dataacquisition unit is configured to apply, precedently to the radiofrequency excitation pulses and for controlling the contrast, afrequency-selective radio frequency pre-pulse, which is applied to amatter having a resonance frequency different from that of NMR signalsacquired for imaging, or a slice-selective radio frequency pre-pulse,which is applied in a slice different from an imaging slice, followed bya spoiler gradient magnetic field.
 5. A magnetic resonance imagingapparatus of claim 1, wherein the one or more radio frequencyintermediate pulses are of the same type.
 6. A magnetic resonanceimaging apparatus of claim 1, wherein the one or more radio frequencyintermediate pulses are of different types.
 7. A magnetic resonanceimaging apparatus of claim 1, wherein each of the spoiler gradientmagnetic fields is applied in a predetermined readout direction.
 8. Amagnetic resonance imaging apparatus of claim 1, wherein said dataacquisition unit is configured to apply each of spoiler gradientmagnetic fields in at least one of a slice direction and a phase encodedirection and to apply a gradient magnetic field for cancelling adephasing amount by the spoiler gradient magnetic field, the gradientmagnetic field for cancelling the dephasing amount being appliedprecedently to the respective radio frequency intermediate pulse.
 9. Amagnetic resonance imaging apparatus of claim 1, wherein said dataacquisition unit is configured to determine a flip angle of each of theradio frequency intermediate pulses depending on the flip angle of therefocus pulses.
 10. A magnetic resonance imaging apparatus of claim 1,wherein said data acquisition unit is configured to transmit the one ormore radio frequency intermediate pulses with plural element coils. 11.A magnetic resonance imaging apparatus of claim 1, wherein said dataacquisition unit is configured to apply the one or more radio frequencyintermediate pulses at a timing according to at least one of an echointerval and an echo time.
 12. A magnetic resonance imaging apparatus ofclaim 1, where a plurality of readout gradient magnetic field pulses areapplied before and after predetermined ones of the radio frequencyintermediate pulses.
 13. A magnetic resonance imaging apparatus of claim12, where a predetermined set of the readout gradient magnetic fieldpulses has a predetermined total amount of time.
 14. A magneticresonance imaging apparatus of claim 13, where among the predeterminedset of the readout gradient magnetic field pulses, the readout gradientmagnetic field pulses applied after the predetermined ones of the radiofrequency intermediate pulses are the spoiler gradient magnetic filedpulses.
 15. A magnetic resonance imaging method comprising: performingan imaging scan on a Fast Spin Echo sequence, the imaging scan includingapplying at least one radio frequency excitation pulse for acquiringmagnetic resonance signals; applying a plurality of refocus pulses aftereach radio frequency excitation pulse; for controlling contrast,applying one or more frequency-selective or slice-selective radiofrequency intermediate pulses, wherein each of the one or more radiofrequency intermediate pulses is separately applied between a respectivepair of successive refocus pulses, and the one or morefrequency-selective radio frequency intermediate pulses are radiofrequency pulses applied to a matter having a resonance frequencydifferent from that of NMR signals acquired for imaging and the one ormore slice-selective radio frequency intermediate pulses are radiofrequency pulses applied in a slice different from an imaging slice; forsuppressing a signal component, applying a spoiler gradient magneticfield subsequent to each of the radio frequency intermediate pulses; andacquiring magnetic resonance signals for imaging; and generating imagedata based on the magnetic resonance signals.